Vacuum pumps

ABSTRACT

In a vacuum pump of the getter-ion type, particularly one using Penning discharges, the problem of &#39;&#39;&#39;&#39;argon instability&#39;&#39;&#39;&#39; is at least partially solved by making at least one of the cathodic surfaces of two different materials. One of the materials may be titanium or like reactive metal, while the other material should also be refractory and of considerably higher atomic mass.

United States Patent Laurenson et' al.

1 51 Jan. 30, 1973 VACUUM PUMPS [56] References Cited Inventors:Laurence Laurenson, Horsham; UNITED STATES PATENTS Lesl'e Arthur wand,3,112,863 12/1963 Brubakeretal ..417 49 both of England 3,018,944 l/l962Zaphiropoulous ..417 49 Assignee: The British Oxygen Company i i London,England Primary Examiner-William L. Freeh Assistant Examiner.l0hn T.Winburn 1970 Attorney-Dennison, Dennison, Townshend & Appl. No.: 89,427Meserole 57 AB TRACT Foreign Application Priority Data I S In a vacuumpump of the getter-ion type, particularly Nov. 14, I969 Great Bntam..55,998/69 one using Penning discharges, the problem of argoninstability is at least partially solved by making at u.s.c1. ..417/49ease ene'ef the eeehedle Surfaces 0f different 1m. c1...............:::III...............:.::..F04b 37/02 meterieeone efthematerials may be titanium er like F d of Search "417/48 49 1 313/338reactive metal, while the other material should also be refractory andof considerably higher atomic mass. Y

4 Claims, 4 Drawing Figures .IAI IO VACUUM, PUMPS This invention relatesto vacuum pumps, and particularly to the type known as Penning pumps.These are getter-ion pumps which trap gas by sorption processes by usinga large number of Penning discharges.

When a Penning pump is used to remove inert gases, such as argon, fromevacuated spaces the phenomenon of argon instability arises. In this,large amounts of argon are released intermittently from the electrodes,causing a rapid and large fluctuation in the pressure of the system.These pressure variations are normally unacceptable and therefore stepshave to be taken against them.

The present invention aims at providing an improved Penning pump inwhich the risk of argon instability is largely reduced or eliminated. 4

Accordingly the present invention provides a Penning pump which is asclaimed in the appended claims.

The present invention will now be described by way of example withreference to the accompanying drawing, in which:

FIG. 1 is a diagrammatic side view of a Penning pump of the presentinvention with one cathode removed;

FIG. 2 is a side view, displaced by 90, of the pump shown in FIG. 1',

FIG. 3 is a section along the line llllll in FIG. 1 of the respectivecathode, and

FIG. 4 is a diagrammatic cross-section, drawn to a larger scale thanthat of FIG. 3, of an alternative form of cathode.

A conventional Penning pump employs two cathodes of titanium bracketinga multi-cellular anode. This form is known to develop pumpinginstabilities when sorbing pure inert gases. This instability can beremoved by substituting for one of the cathodes a cathode made oftantalum. This considerably increases the cost of the pump, and also,because tantalum is less reactive than titanium, the speed of the pumpfor pumping active gas is appreciably lower than that using two cathodesof titanium. The pump of the present invention is able to pump activegases and yet is not subject to argon instability to the same extent asa Penning pump using two titanium cathodes.

A pump of the present invention has its essential components shown inFIGS. 1 and 2. It includes an anode 2 positioned between two cathodes 4.The anode 2 is made up of an array of cylindrical cells 6 of stainlesssteel which are joined together to produce a honeycomb arrangement, ascan be most clearly seen from FIG. 1. The cathodes 4 are made primarilyof titanium, but at least one of the cathodes has part of its surfacearea 8 made of tantalum. As shown in FIG. 1, the tantalum may be intheform of a strip of metal, but it could alternatively be applied tothe surface of the cathode 4 as a series of discs or other separatepieces of tantalum.

For clarity, the means are omitted by which the electrodes are connectedto their associated electrical circuitry.

The manner in which the tantalum is secured to the cathodes does notform part of the present invention, but as a feature of the inventionthe tantalum may be detachable so that it can be conveniently replacedor alternatively removed altogether when it is desired to use the pumpas a known Penning pump having two wholly-titanium cathodes, with theconsequent risk of argon instability.

FIG. 3 of the accompanying drawing illustrates diagrammatically how thestrip of tantalum 8 is positioned in front of the respective major faceof the titanium cathode 4.

In known Penning pumps the opposed major faces of the cathodes 4 may begrooved, as illustrated in FIG. 4. This has been found to increase theeffectiveness of the pump for pumping inert gases, in that the ionsthereof tend to get buried in the bottoms of the grooves by thedeposition of sputtered electrode material. Experiments have shown thata pump is far more stable when pumping argon if tantalum strips areplaced in front of plain cathodes. This improvement is even more markedwhen such strips are placed in front of slotted-face cathodes.

The manner in which the present invention works is not'completely clear,but it would seem that the most likely explanation is given by Jepsensenergetic neutral theory. Briefly, this theory suggests that as theenergized ions of a gas such as argon impinges on a material which is asheavy as tantalum, the ions tend to bounce back from the tantalumwithout burying themselves below its surface. During this reflectionphase the ion is discharged so that the cathode now produces a supply ofneutral species still having a high energy. These electrically neutralparticles are then able to embed themselves in the anode, becomingcovered with further sputtered material. It will be appreciated thatsome inert gas will still be taken up on the cathode, but the amountembedded in a tantalum cathode will be appreciably lower than that in atitanium cathode operated under the same conditions. This means that notall the pumping of the inert gas has been transferred to the anode, butonly that the concentration of argon embedded or buried in the cathodehas been reduced to what appears to be a stable level.

One of the features of this invention is that'only one of the cathodesneed have its respective surface formed partly of tantalum or likematerial. Symmetrical cathodes can of course be used, but it is feltthat these do not offer sufficient additional advantages over theasymmetrical arrangement to merit the additional cost of the tantalum orlike metal. It will then seem that if a pump of the present inventionused one cathode made wholly of titanium, then the pump would still beargon stable. A tentative explanation for the non-appearance of thisphenomenon in a pump of the present invention is that although the inertgas is not pumped stably at the titanium cathode, any reemission ofargon from the titanium cathode can immediately be taken up byreionization and transference into the tantalum cathode/cellular anodepumping combination. Because the reemitted argon is so quickly absorbedby this pumping combination elsewhere in the pump, then to an observerviewing a vacuum gauge connected to the pump, the pressure remainsconstant in the pump and the system being evacuated.

The applicants put forward the above explanation only tentatively, anddo not wish to be committed to a particular theory of operation.

An alternative material for the main part of the cathode can bezirconium, which is sufficiently reactive and refractory.

The cathode materials dissimilar to titanium are chosen from a groupconsisting of tantalum, tungsten, molybdenum, zirconium, niobium andrhenium.

We claim:

l. A getter-ion pump of the Penning type, including a pair of laterallyspaced parallel imperforate cathodes, a

multi-cellular anode positioned between the cathodes and spacedtherefrom, at least one cathode having a major surface facing the anode,being constituted and arranged to provide a plurality of slots therein,said major surface having fixed thereon a separate portion constitutedof a refractory metal dissimilar to the metal of said major surface andchosen from a group consist-

1. A getter-ion pump of the Penning type, including a pair of laterallyspaced parallel imperforate cathodes, a multi-cellular anode positionedbetween the cathodes and spaced therefrom, at least one cathode having amajor surface facing the anode, being constituted and arranged toprovide a plurality of slots therein, said major surface having fixedthereon a separate portion constituted of a refractory metal dissimilarto the metal of said major surface and chosen from a group consisting oftantalum, molybdenum, zirconium, niobium and rhenium.
 2. A pump asclaimed in claim 1 in which the metal comprising said major surface isconstructed and arranged to provide a plurality of grooves therein.
 3. Apump as claimed in claim 1 in which a portion of the slots is spanned bya covering strip of said dissimilar metal.